The present invention relates to a method of making a polarizing article from a phase-separated glass containing silver, copper, or copper-cadmium halide crystals.
Over the years many have developed and practiced various techniques for precipitating silver, copper, or copper-cadmium halide crystals by heat treating glasses with compositions containing the respective metals and a halogen other than fluorine, in suitable amounts. The glasses that result usually exhibit photochromic behavior, that is, they darken and fade in color, respectively, in response to the application and removal of short wavelength radiation. It is possible, however, to produce glasses which contain the indicated crystals, but which are not photochromic.
Stretching the glass within a certain viscosity range can generate a birefringent effect in these crystal-containing glasses. The glass is placed under stress at a temperature above the glass strain point temperature. This elongates the glass, and thereby elongates and orients the crystals within. The elongated article is then exposed to a reducing atmosphere at a temperature above 250xc2x0 C., but not exceeding 25xc2x0 C. above the glass annealing point. This develops a surface layer in which at least a portion of the halide crystals is reduced to elemental metal. The elongated elemental crystals provide an array of electric dipoles that preferentially interact with the electric field vector of incident light. This provides a method to polarize light waves that are transmitted through it.
The production of a polarizing glass involves, broadly, four basic steps:
1. Melting a glass batch containing a source of silver, copper, or copper-cadmium and a halogen other than fluorine, and forming a body from the melt;
2. Heat treating the glass body at a temperature above the glass strain point to generate halide crystals having a size in the range of 200-5000xc2x0 C.;
3. Stressing the crystal-containing glass body at a temperature above the glass strain point to elongate the body and thereby elongate and orient the crystals; and
4. Exposing the elongated body to a reducing atmosphere at a temperature above 250xc2x0 C. to develop a reduced surface layer on the body that contains metal particles with an aspect ratio of at least 2:1.
The glass articles produced according to the general method tend to display excellent polarizing properties over the infrared portion of the radiation spectrum, preferably within the region of 600-2000 nm (6000-20,000 xc3x85). Typically, stretching the glass within a certain viscosity range can generate a birefringent effect in these crystal-containing glasses. The glass is placed under stress at a temperature above the glass strain point temperature. The stress elongates the glass, and thereby elongates and orients the crystals. The elongated article is then exposed to a reducing atmosphere at a temperature above 250xc2x0 C., but not to exceed over 25xc2x0 C. above the glass annealing point. The reducing atmosphere develops a surface layer in the glass in which at least a portion of the halide crystals is reduced to elemental metal. The elongated elemental metals provide an array of electric dipoles, which preferentially reacts with the electric field vector of incident light. This phenomenon provides a method to polarize transmitted light waves.
The growth of halide particles cannot occur at temperatures below the strain point of the glass because the viscosity of the glass is too high. Therefore, temperatures above the annealing point are preferred for crystal precipitation. Where physical support is provided for the glass body, temperatures up to 50xc2x0 C. above the softening point of the glass can be employed.
Experience has demonstrated that the halide crystals should have a diameter of at least about 200 xc3x85 in order to assume upon elongation, an aspect ratio of at least 5:1. When reduction to elemental particles occurs, the particles having an aspect ratio of at least 5:1 will display an aspect ratio greater than 2:1. This places the long wavelength peak at least near the edge of the infrared region of the radiation spectrum, while avoiding serious breakage problems during the subsequent elongation step. At the other extreme, the diameter of the initial halide particles should not exceed about 5000 xc3x85. This precludes the development of significant haze in the glass accompanied with a decreased dichroic ratio resulting from radiation scattering.
The dichroic ratio is a measure of the polarizing capability of a glass. It is defined as the ratio existing between the absorption of radiation parallel to the direction of elongation and the absorption of radiation perpendicular to the direction of elongation. To attain an adequate ratio, the aspect ratio of the elongated halide crystals must be at least 5:1 so that the reduced metal particles have an aspect ratio of at least 2:1.
Crystals having a small diameter demand very high elongation stresses to develop a necessary aspect ratio. Also, the likelihood of glass body breakage during a stretching-type elongation process is directly proportional to the surface area of the body under stress. This creates a very practical limitation as to the level of stress that can be applied to a glass sheet, or other body of significant mass. In general, a stress level of a few thousand psi has been deemed to comprise a practical limit, but often stress levels above 3000 psi are customarily used.
One of the key measures of the effectiveness of a polarizing glass body is its contrast ratio, or simply its contrast, as referred to in the art. Contrast comprises the ratio of the amount of radiation transmitted with its plane of polarization perpendicular to the elongation axis to the amount of radiation transmitted with its plane of polarization parallel to the elongation axis. In general, the greater the contrast, the more useful, and valuable, the polarizing body. Another important feature of a polarizing body is the bandwidth over which the body is effective. This property takes into consideration not only the degree of contrast, but also the portion of the spectrum within which the contrast is sufficiently high to be useful.
The level of contrast attainable in a polarizing glass body is dependent upon the amount of reduction occurring during the step of firing in a reducing atmosphere. Typically, the greater the extent of reduction the greater the level of contrast. Thus, employing a combination of either higher temperatures, longer times, or higher pressures of reducing gas species for reduction, can increase the degree of contrast.
That practice is limited, however, by the tendency of the metal halide particles to respheriodize. Firing of the elongated body in a reducing atmosphere is undertaken at temperatures above 250xc2x0 C., but no higher than 25xc2x0 C. above the annealing point of the glass. Preferably, the firing temperature is somewhat below the annealing point of the glass to prevent the particles from respheriodizing. Respheriodization or the tendency for the elongated particles to return to their original state, or to break into small particles, arises as the temperature of the reduction step, or of any other heat treatment subsequent to the elongation step, is increased. This tendency places a serious limitation on the temperature at which any such subsequent heat treatment may be undertaken.
That tendency is also enhanced by higher temperatures and longer times of firing. Respheriodization can result in a decrease in contrast and/or a narrowing of the peak absorption band, or a shifting of the peak absorption band in the direction of shorter wavelengths. To illustrate, a process for preparing polarizing glass articles in accordance with prior knowledge has utilized firing in a hydrogen atmosphere for 4 hours at 425xc2x0 C. When the firing time was extended to 7 hours, the contrast was increased somewhat, but with a concurrent reduction in the bandwidth of high contrast.
U.S. Pat. No. 4,908,054 (Jones et al.) proposes a method of producing a polarizing glass body that obviates the effect of respheriodization during a heat treatment such as the reduction step. This method conducts the thermal reducing treatment under a pressure of at least twice atmospheric pressure. The effect of the pressure is to inhibit respheriodization and to produce a polarizing glass article that exhibits a relatively broad range of high contrast polarizing properties in the infrared region. This expedient is not required in the present invention, but may be employed.
It is a purpose of the present invention to provide a glass article that has excellent polarizing properties over a wide range of the radiation spectrum. Another purpose is to accomplish this with or without employing the Jones et al. expedient of enhanced pressure. A further purpose is to provide a phase-separated glass that is elongated at a relatively low stress level. A still further purpose is to produce a polarizing glass article having a relatively flat contrast absorption curve over a wide wavelength band.
The invention encompasses a method of producing a glass article that exhibits a broad band of high contrast polarizing properties in the infrared region of the radiation spectrum. The glass is phase-separated, or exhibits photochromic properties, based on silver, copper, or copper-cadmium halide crystals precipitated in the glass having a size within a range of 200-5000xc3x85, and contains elongated silver, copper, or copper-cadmium metal particles. The method comprises thermally forming and precipitating large halide crystals in the glass article by subjecting the glass to a time-temperature cycle in which the temperature is about at least 76xc2x0 C. or greater above the glass softening point, and the time is sufficient to form the crystals, preferably over one hour. Preferably the temperature applied is higher than 76xc2x0 C., such as about 77xc2x0 C. to about 80xc2x0 C. above the glass softening point. The glass article, halide crystals, and particles within are elongated at a temperature between the strain point and the softening point of the glass.
The invention further encompasses a method for making a glass article exhibiting a relatively broad band of high contrast polarizing properties in the infrared region of the radiation spectrum from glasses which are phase-separable, or exhibit photochromic properties, through the presence of silver, copper, or copper-cadmium halide crystals, the method comprising the steps of:
(a) melting a batch for a glass containing a source of silver, copper, or copper-cadmium and at least one halogen other than fluorine or a combination thereof,
(b) cooling and shaping the melt into a glass article of a desired configuration,
(c) subjecting the glass article to a temperature at least about 76xc2x0 C. above the softening point of the glass to generate and precipitate silver, copper, or copper-cadmium crystals in the glass, the crystals ranging in size between about 200 xc3x85 and 5000 xc3x85,
(d) elongating the glass article under stress not over about 3000 psi (preferably not over about 2500-2675 psi) at a temperature above the strain point of the glass to provide the crystals with an elongated aspect ratio of at least 5:1 and aligned in the direction of the stress, and,
(e) exposing the elongated glass article to a reducing atmosphere at a temperature above about 250xc2x0 C., but no higher than about 25xc2x0 C. above the annealing point of the glass for a period of time sufficient to develop a reduced surface layer on the glass article wherein at least a portion of the elongated halide crystals are reduced to elemental silver, copper, or copper-cadmium particles having aspect ratios greater than 2:1 which are deposited in and/or upon said elongated crystals, whereby the glass article exhibits a relatively broad range of high contrast polarizing properties in the infrared region of the radiation spectrum.